Graded liquid crystal polymer package

ABSTRACT

A device ( 10 ) is provided for matching the CTE between substrates ( 12, 14 ), e.g., a semiconductor substrate and packaging material. The first substrate ( 12 ) has a first coefficient of thermal expansion and the second substrate ( 14 ) has a second coefficient of thermal expansion. At least two layers ( 16 ) of liquid crystal polymer are formed between the first substrate ( 12 ) and the second substrate ( 14 ), each layer having a unique coefficient of thermal expansion progressively higher in magnitude from the first substrate ( 12 ) to the second substrate ( 14 ).

FIELD OF THE INVENTION

The present invention generally relates to integrated circuit packaging,and more particularly to attaching an integrated circuit to a substrate.

BACKGROUND OF THE INVENTION

With the growth of the use of personal communication devices, e.g., cellphones and two way radios, high performance and high frequency packagingmaterials have increased in importance. Desired characteristics forelectronic packaging include high electric and thermal performance,thinness, low weight, small size, high component density, and low cost.

When attaching an integrated circuit to a packaging material, e.g., aprinted circuit board or a polymer material, it is known that thecoefficient of thermal expansion (CTE) of the integrated circuit and thepackaging material must be matched. When the CTE of the two materialsare matched, the two materials will expand and contract simultaneouslyover temperature so as to avoid deformities, cracking, detachment, andloss of functionality, especially after a number of temperature cycles.The importance of this matched CTE becomes apparent in many applicationshaving large temperature swings, e.g., automotive electronics.

Conventional packages are fabricated from materials such as plastic,Teflon or ceramics. The type of material that is used depends on anumber of factors which include frequency of operation, environment andcost. Plastic packages are typically the lowest in cost but may not besuitable for high frequencies of operation or very high temperatures.Applications that require exposure to extreme temperatures will commonlyuse ceramics. The metallization that can be used will typically differdepending on the packaging material. As the frequency of operationincreases, factors such as surface roughness and metal thickness becomeimportant. In addition to these factors, as the frequency of operationincreases it becomes advantageous to utilize materials that have lowerdielectric constants to allow for the implementation so that the finalpackage, with interconnects, will avoid noise or signal loss associatedwith high speed circuits

One known solution is to place the integrated circuit on the substrateand within a hole formed in a liquid crystal polymer material; however,this adds complexity to the manufacturing process.

Another known solution involves the formation of a single layer ofliquid crystal polymer between two non-liquid crystal polymersubstrates; however, this results in layers that will not have as good aperformance at a high frequency as liquid crystal polymer.

Accordingly, it is desirable to provide a liquid crystal polymer packagethat matches the CTE of an integrated circuit to that of the packagingmaterial. Furthermore, other desirable features and characteristics ofthe present invention will become apparent from the subsequent detaileddescription of the invention and the appended claims, taken inconjunction with the accompanying drawings and this background of theinvention.

BRIEF SUMMARY OF THE INVENTION

A device is provided for matching the CTE of two substrates. The devicecomprises a first substrate having a first coefficient of thermalexpansion and a second substrate having a second coefficient of thermalexpansion. At least two layers of liquid crystal polymer are formedbetween the first substrate and the second substrate, each layer havinga unique coefficient of thermal expansion progressively higher inmagnitude from the first substrate to the second substrate. The firstand second substrates may comprise a semiconductor substrate andpackaging material.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe following drawing figures, wherein like numerals denote likeelements, and

FIG. 1 is a graph demonstrating the moisture barrier properties of thematerial used in an exemplary embodiment of the present invention.

FIG. 2 is a cross-sectional view of an exemplary embodiment of thepresent invention taken along the line 3-3 of FIG. 3; and

FIG. 3 is a top cross-sectional view taken along the line 2-2 of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of the invention is merely exemplaryin nature and is not intended to limit the invention or the applicationand uses of the invention. Furthermore, there is no intention to bebound by any theory presented in the preceding background of theinvention or the following detailed description of the invention.

Liquid crystal polymer (LCP) combines the properties of polymers withthose of liquids and provide superior thermal and electrical propertiesincluding low loss, low dielectric constant, and low coefficient ofthermal expansion (CTE) characteristics. LCP packages are especiallyadvantageous for RF devices due to their low signal toss and lowdielectric constant (3.01 at 1 MHz) over a wide frequency range andsuperior moisture barrier properties (0.04% water absorption). FIG. 1shows that LCP 4 demonstrates superior moisture barrier properties ascompared with two other conventional substrate materials, an organicmaterial 6 and polyimide 8, relative to loss tangent. The dielectricconstant is important for RF packaging because it determines thecharacteristic impedance of the circuitry, which relates to size and tothe signal loss of the circuitry. Loss tangent is important and directlyrelated to circuit signal losses and Q factor. Q factor is a figuremerit in filters, resonators, and low noise circuits.

LCP is an ordered thermoplastic polymer with long stiff molecules thatoffer an excellent combination of electronic, thermal, mechanical andchemical properties that make them an excellent material choice forelectronic packaging. LCPs are highly crystalline materials based onaromatic ring-structured compounds that are very stable afterpolymerizing. Characteristics of a particular LCP depend on themanufacturer, but exist in a variety of unfilled, glass-filled,mineral-filled, carbon fiber reinforced, and glass fiber-reinforcedgrades that allow for numerous options in key properties such as theCTE. LCP laminates have a CTE that can be readily matched to that ofsilicon and other materials. Also, the high moisture and chemicalresistance improve LCP performance in unfriendly operating environments,and the low CTE, low dielectric constant, and high dielectric strengthmake it desirable as circuit board laminates for electronics packaging.Furthermore, LCP has a high moisture barrier which may be used to sealand protect electronic components from high humidity.

In an exemplary embodiment and referring to the device 10 of FIG. 2, across sectional view is shown as viewed along the line 3-3 of FIG. 3.The CTE of the first substrate 12 is lower than the CTE of the secondsubstrate 14 so that attaching them directly together would causedeformities or cracks, for example, in one or both of the firstsubstrate 12 or second substrate 14. In general, graded layers 16 of LCPprovide a transition in CTE between the CTE of first substrate 12 andthe CTE of second substrate 14. The material for each of the gradedlayers 16 can be selected, as desired, for a particular first substrate12 material and a particular second substrate 14. More particularly,each of the graded layers 16 are selected such that the CTE is“stair-stepped” from the first substrate 12 to the second substrate 14.

More particularly, the graded layers 1-6 comprise a LCP wherein the CTEof layer 22 adjacent to the first substrate 12 is closely matched to theCTE of the first substrate 12, but slightly higher. The layer 24adjacent layer 22 is closely matched thereto; however the CTE of layer24 is slightly higher than the CTE of layer 22. Each successive layer ofthe graded layers has a CTE closely matching that of the adjacent layer,but the CTE of each of the graded layers increases as it gets closer tothe second substrate 14. The CTE of the layer 28 adjacent to the secondsubstrate 14 is closely matched to the CTE of the second substrate 14.Effectively, this approach reduces the stress build up at any onelayer-to-layer interface by distributing it across multiplelayer-to-layer interfaces.

In the exemplary embodiment shown in FIG. 2, one of the first or secondsubstrates 12, 14 may comprise, for example, an integrated circuitsubstrate comprising silicon, but may alternative comprise any type ofmaterial used for integrated circuits, such as germanium,silicon/germanium, or a III-V compound. The other of the first or secondsubstrates 12, 14 may comprise, for example, packaging materialcomprising any type of material used for electronic packaging, such asthat used for printed circuit boards, a polymer, glass, etc. Thethickness of the graded layers 16 may range from 1 mil to 30 mils;however the thickness of the graded layers 16 is dependent upon thefrequency needed for a particular application.

An example of the thicknesses and CTEs for the first substrate 12,second substrate 14 and graded layers 16 is illustrated in the chart asfollows: MATERIAL THICKNESS (mils) CTE (ppm/C.) first substrate 12 242.6 layer 22 2 3.5 layer 24 2 4.4 layer 26 2 5.3 layer 28 2 6.2 secondsubstrate 14 125 7

A via 32 may be formed through the graded layers 16 in a manner known tothose in the industry for providing electrical connection betweencircuitry on the first substrate 12 and circuitry on the secondsubstrate 14. A via 34 also formed in the graded layers 16 may makeelectrical contact with circuitry on the integrated substrate by a wirebond 36. A via 38 may be formed through layers 22 and 24 to terminate atthe junction 40 between layers 24 and 26. Another via 42 may be formedthrough layers 26 and 28 to also terminate at the junction 40.

Referring to FIG. 3, the device 10 is shown as viewed along line 2-2 ofFIG. 2. Functional circuitry 44 is formed on the surface 40 of layer 26in a manner known to those in the industry. Functional circuitry 44 maycomprise any type of electronic circuitry, for example, a microstriptransmission line as shown, a coplanar waveguide, a resistor, aninductor, or filtering structures. A first end 46 of the functionalcircuitry 44 is connected to the via 42 and a second end 48 would beconnected to the via 38.

LCP layers would be manufactured in sheet form using standard processesknown to the industry. A selection of off-the-shelf and/or customizedCTE LCP layers would be made for a particular application. The layerswould be laminated using interleaved adhesive layers or alternatingsingle sided metalized LCP layers, or other standard method inconjunction with the proper heat and pressure to achieve proper bonding.

While at least one exemplary embodiment has been presented in theforegoing detailed description of the invention, it should beappreciated that a vast number of variations exist. It should also beappreciated that the exemplary embodiment or exemplary embodiments areonly examples, and are not intended to limit the scope, applicability,or configuration of the invention in any way. Rather, the foregoingdetailed description will provide those skilled in the art with aconvenient road map for implementing an exemplary embodiment of theinvention, it being understood that various changes may be made in thefunction and arrangement of elements described in an exemplaryembodiment without departing from the scope of the invention as setforth in the appended claims.

1. A device comprising: a first substrate having a first coefficient ofthermal expansion; a second substrate having a second coefficient ofthermal expansion higher in magnitude than the first coefficient ofthermal expansion; and at least two layers of liquid crystal polymerformed between the first substrate and the second substrate, each layerhaving a unique coefficient of thermal expansion progressively higher inmagnitude from the first substrate to the second substrate.
 2. Thedevice of claim 1 wherein one of the first and second substratescomprises a packaging material.
 3. The device of claim 1 wherein one ofthe first and second substrates comprises an integrated circuit.
 4. Thedevice of claim 3 wherein integrated circuit comprises at least one ofsilicon, germanium, and a III-V material.
 5. The device of claim 1wherein one of the first and second substrates comprises one of aprinted circuit board, a polymer material, and a ceramic material. 6.The device of claim 1 wherein each of the two layers comprise athickness of between 1 and 30 mils.
 7. The device of claim 1 wherein atleast one via is formed from the first substrate to the second substratethrough the at least two layers.
 8. The device of claim 1 furthercomprising: an electronic device formed between the at least two layers;a first via formed between the first substrate and the electronicdevice; and a second via formed between the second substrate and theelectronic device.
 9. A device comprising: a semiconductor substratehaving a first coefficient of thermal expansion; a packaging materialhaving a second coefficient of thermal expansion; and a plurality oflayers of liquid crystal polymer formed between the semiconductorsubstrate and the packaging material, each layer having a uniquecoefficient of thermal expansion progressively higher in magnitude fromthe first substrate to the second substrate.
 10. The device of claim 9wherein integrated circuit comprises at least one of silicon, germanium,and a III-V material.
 11. The device of claim 9 wherein one of the firstand second substrates comprises one of a printed circuit board, apolymer material, and a ceramic material.
 12. The device of claim 9wherein each of the two layers comprise a thickness of between 1 and 30mils.
 13. The device of claim 9 wherein at least one via is formed fromthe first substrate to the second substrate through the at least twolayers.
 14. The device of claim 9 further comprising: an electronicdevice formed between the at least two layers; a first via formedbetween the first substrate and the electronic device; and a second viaformed between the second substrate and the electronic device.
 15. Amethod of making a device that matches the coefficient of thermalexpansion between a first material having a first coefficient of thermalexpansion and a second material having a second coefficient of thermalexpansion, comprising: forming a first layer of liquid crystal polymeradjacent the first material; and forming a second layer of liquidcrystal polymer between the second material and the first layer, thefirst and second layers having a unique coefficient of thermalexpansion, the magnitude of the coefficient of thermal expansion of thesecond material being greater than that of the second layer, the secondlayer being greater than that of the first layer, and the first layerbeing greater than that of the first material.
 16. The device of claim15 wherein one of the first and second material comprises an integratedcircuit.
 17. The device of claim 15 wherein one of the first and secondmaterial comprises a packaging material.
 18. The device of claim 15wherein each of the first and second layers comprise a thickness ofbetween 1 and 30 mils.
 19. The device of claim 15 wherein at least onevia is formed from the first material to the second material through theat least two layers.
 20. The device of claim 15 further comprising: anelectronic device formed between the at least two layers; a first viaformed between the first substrate and the electronic device; and asecond via formed between the second substrate and the electronicdevice.